1. Field of the Invention
This invention relates generally to an electric drilling machine, and more particularly to a dual-motor-driven drilling machine equipped with an electric control system. The invention further relates to a method of controlling currents supplied to in built-in motors of the drilling machine.
2. Description of the Prior Art
Drilling machines of the type relevant to the present invention often include two motors. One is a drill motor for rotating a cylindrical cutter and the other is a feed motor for feeding the cylindrical cutter during drilling. In conventional drilling machines, the feed speed of the drilling unit is controlled so that the rotation speed of the cylindrical cutter is held at a fixed value when drilling. In other words, the current flowing through the feed motor is controlled so that the current flowing through the drill motor maintains a fixed value.
On the other hand, different types of cylindrical cutters are selectively used depending on such factors as the material to be drilled, the diameter of the hole desired and the depth of the hole desired. When using cylindrical cutters that are large diametered, long, or both, the fixed rotation speed is too high for such cutters to execute drilling, so the drill motor is liable to quickly overload. Contrarily, when using cylindrical cutters that are small diametered, short or both, the fixed rotational speed is too slow for such cutters so that drilling is less than optimally efficient.
Other problems with existing drilling machines include chatter vibration that is sometimes generated at the cylindrical cutter when drilling starts, dispersion of swarf and water from the object being drilled, difficulty in drilling at desired angles and difficulty in removing the drill bit from the object drilled once drilling is completed.